Thorium

• 1. Introduction

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  Thorium, element number 90, has an isotope called thorium-232 whose half-life is 14.0 billion years. That is roughly the age of the universe itself. The metal is soft and bright silver, and it tarnishes olive grey when it meets air. Ground to a fine powder, it can ignite on its own. Yet this slowly decaying element is estimated to be over three times as abundant in the Earth's crust as uranium. Its story begins with a mistake. It later lit rooms through gas mantles, ended up in camera lenses, and was even injected into patients. Today governments still debate whether it could fuel a new generation of reactors. How did one element end up in so many roles? Why was it announced twice? And why has a metal this plentiful stayed so far in the background?

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• 2. Two Discoveries

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  In 1815, the Swedish chemist Jöns Jacob Berzelius examined an unusual sample of gadolinite from a copper mine in Falun, central Sweden. He saw traces of a white mineral in it and cautiously guessed that it was an oxide of an unknown element. Berzelius had already discovered cerium and selenium. He had also once announced an element called gahnium, which turned out to be zinc oxide. In 1817 he privately named the suspected element thorium, and its oxide thorina, after Thor, the Norse god of thunder. Then, in 1824, more deposits of the same mineral turned up in Vest-Agder, Norway. Berzelius retracted his claim. The mineral, later named xenotime, was mostly yttrium orthophosphate.

  In 1828, a black mineral turned up on Løvøya island, in Telemark county, Norway. Its finder was Morten Thrane Esmark, a priest who served as vicar in Telemark and studied its minerals as an amateur. He habitually mailed the most interesting specimens to his father, Jens Esmark. The elder Esmark was a noted mineralogist and a professor at the Royal Frederick University in Christiania, today called Oslo. He could not match the rock to any known mineral, so he sent a sample to Berzelius. This time the chemist concluded that it held a new element. He published his findings in 1829, having isolated an impure sample by reducing potassium pentafluorothorate(IV) with potassium metal. He reused the old name for the element and called the source mineral thorite.

  Berzelius got some things right and one thing wrong. He measured the mass ratio of thorium to oxygen in the oxide at 7.5, close to the true value of about 7.3. But he took the element to be divalent rather than tetravalent. That error put his atomic mass at 7.5 times oxygen's, or 120, when the true figure is 15 times oxygen's. Pure metal had to wait much longer. In 1914, the Dutch entrepreneurs Dirk Lely Jr. and Lodewijk Hamburger isolated metallic thorium for the first time.

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• 3. Born in Stars

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  Thorium-232 was formed in the r-process, short for rapid neutron capture, which probably takes place in supernovae and neutron star mergers. Those violent events scattered the element across the galaxy. Stars cannot build elements heavier than iron by ordinary fusion, because fusing past iron-56 absorbs energy instead of releasing it. Neutron capture is the only route. Stability drops abruptly past bismuth-209, so the r-process is the only stellar process that can make thorium and uranium.

  Across the universe, thorium ranks 77th in abundance, which makes it one of the rarest primordial elements. Only thulium, lutetium, tantalum, rhenium and uranium are rarer. Part of the reason is that thorium has been decaying away since the moment it formed.

  On Earth the picture changes. Thorium is the 37th most abundant element in the crust, at 12 parts per million. It tends to form oxide minerals that stay in the crust instead of sinking into the core, which is why chemists classify it as a lithophile. Because it is only a minor constituent of most minerals, people long assumed it was rare.

  Its decay also warms the planet. Thorium-232 is the largest single contributor to the Earth's internal heat, followed by uranium-238, potassium-40 and uranium-235. Because of that slow decay, the planet now holds about 80% of the thorium it started with.

  Monazite, a yellow or brown sand made mostly of rare-earth phosphates, is the main commercial source. Large deposits lie in India, South Africa, Brazil, Australia and Malaysia. The sand averages around 2.5% thorium, and some deposits reach 20%. It is chemically unreactive, which makes the thorium hard to extract. Thorite, the mineral in which thorium was first found, holds more thorium still. Radiation from the element wrecks the crystal structure of the minerals that contain it. Ekanite almost never occurs in an undamaged form for exactly this reason.

  Demand is so low that a mine cannot pay its way on thorium alone, so the metal is nearly always a by-product of rare-earth extraction. In 2014, world production of monazite concentrate was 2,700 tonnes. A table of lower-bound reserve estimates from that year totals 6,355 thousand tonnes worldwide.

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• 4. Glowing Mantles

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  In 1885, the Austrian chemist Carl Auer von Welsbach invented the gas mantle, and thorium finally had a job. The mantle was a portable light source. Burning gaseous fuel heated thorium oxide until it glowed. Thorium dioxide suits the task because it has the highest melting point of any known oxide, roughly 3300 degrees Celsius. It stays solid in a flame and makes the flame far brighter. Most substances give off light across a broad spread of wavelengths at high temperature. Thorium's light falls almost entirely in the visible range.

  Adding a little cerium dioxide changes the color. Thorium dioxide on its own gives off an intense blue light when heated in air, and mixing in cerium dioxide turns the light white. In 1901 came a surprise. A hot Welsbach mantle made with thorium dioxide and 1% cerium dioxide stayed at full glow when exposed to a cold, unlit mixture of flammable gas and air. No flame was needed. The effect is called candoluminescence. Thorium dioxide with cerium acts as a catalyst for the recombination of free radicals in a flame, and that releases large amounts of energy as light.

  Since the 1920s, thoriated tungsten wires have gone into electronic tubes and into the cathodes and anticathodes of X-ray tubes and rectifiers. A small addition of thorium lowers the energy electrons need to escape from tungsten, so they are emitted at far lower temperatures. Thorium forms a layer one atom thick on the tungsten surface. Because thorium reacts with oxygen, tube makers added an evaporated magnesium layer to trap impurities, which gave the tubes their characteristic metallic inner coating. Transistors in the 1950s cut into this use without ending it. Thorium dioxide also went into tungsten powder for lamp filaments, where it slows recrystallisation, and into welding electrodes, where it improves arc stability.

  Heat-resistant laboratory crucibles were another use for thorium dioxide, either as the main ingredient or mixed with zirconium dioxide. An alloy of 90% platinum and 10% thorium once served as a catalyst for oxidising ammonia to nitrogen oxides. It lost out to an alloy of 95% platinum and 5% rhodium, which proved more durable. Magnesium alloyed with thorium, known as Mag-Thor, found some aerospace uses.

  Thorium tetrafluoride works as an anti-reflection material in multilayered optical coatings. It is transparent to wavelengths from 0.350 to 12 micrometres, which spans near ultraviolet, visible and mid infrared light. Thorium dioxide in glass raises the refractive index and lowers dispersion, which suits high-quality camera and scientific lenses. Over years, the radiation can darken such lenses and turn them yellow, and it degrades film, though the health risks are minimal. Lengthy exposure to intense ultraviolet light can restore yellowed lenses to colourless.

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• 5. Radiation and Risk

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  In 1898, the German chemist Gerhard Carl Schmidt found that thorium is radioactive. Later that year the Polish-French physicist Marie Curie, working independently, reached the same result. Thorium became the second radioactive element, after the French physicist Henri Becquerel found radioactivity in uranium in 1896.

  From 1899, the New Zealand physicist Ernest Rutherford and the American electrical engineer Robert Bowie Owens studied thorium's radiation and found that it varied significantly. The cause was a short-lived gas given off by thorium, which proved to be a new element. It is now called radon, and it is the only one of the rare radioelements discovered in nature as a daughter of thorium rather than uranium. From 1900 to 1903, Rutherford worked with the British physicist Frederick Soddy and showed that thorium decays at a fixed rate into a series of other elements. That work gave science the idea of the half-life.

  The biological effect of radiation was discovered in 1903, and radioactivity excited scientists and the public alike. In the 1920s, thorium's radioactivity was promoted as a cure for rheumatism, diabetes and sexual impotence. In 1932, after a federal investigation into the health effects of radioactivity, the United States banned most of these uses. By then 10,000 people in the country had been injected with thorium during X-ray diagnosis. They were later found to suffer health problems such as leukaemia and abnormal chromosomes. Public interest in radioactivity faded by the end of the 1930s.

  A 1981 study by the Oak Ridge National Laboratory in the United States estimated that using a thorium gas mantle every weekend would be safe for a person. It was not safe for the people who manufactured mantles, nor for the soils around some factory sites. The reason lies in the decay products. The daughters of thorium have much lower melting points than thorium dioxide, so they volatilise whenever a mantle is heated. In the first hour of use, large fractions of radium-224, radium-228, lead-212 and bismuth-212 escape. Most of a user's dose comes from inhaling radium, up to 0.2 millisieverts per use, about a third of the dose from a mammogram.

  In the 1990s, most uses that did not depend on thorium's radioactivity declined quickly as safer replacements were found. Some mantle makers switched to materials such as yttrium, and rare-earth oxides such as lanthanum took over in lenses. Yet as recently as 2007, some companies still sold thorium mantles without adequate information about their radioactivity. A few even claimed falsely that the mantles were non-radioactive.

  Thorium itself is odourless and tasteless. Its chemical toxicity is low, because the metal and its common compounds barely dissolve in water. Once inside the body, about three-quarters of it accumulates in the skeleton, and the kidneys and faeces clear it slowly. Symptoms can take as much as thirty years to appear after ingestion. Breathing thorium dust raises the risk of cancers of the lung, pancreas and blood. Workers who process monazite were found with thorium above recommended limits, but no adverse health effects appeared at those moderately low levels.

  The metal also carries a risk of fire. In 1956, the Sylvania Electric Products explosion occurred in New York City during the reprocessing and burning of thorium sludge. Nine people were injured, and one died of complications from third-degree burns. In 1964, the United States Department of the Interior listed thorium as "severe" in a table of ignition and explosibility of metal powders. Dust clouds ignited at 270 degrees Celsius.

  The average human body contains about 40 micrograms of thorium and takes in about three micrograms a day, mostly by inhaling dust. In the Tamil Nadu coastal areas of India, residents may receive a natural radiation dose ten times above the worldwide average. On the northern coast of Brazil, from south Bahia to Guarapari, radiation levels reach up to 50 times the world average. Guarapari is a city with radioactive monazite sand beaches. Thorium dust from weapons testing ranges has also been blamed for a high incidence of birth defects and cancer at Salto di Quirra, on the Italian island of Sardinia. Thorium is used in the guidance systems of some missiles.

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• 6. A Misfiled Element

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  Dmitri Mendeleev's periodic table of 1869 put thorium and the rare-earth elements outside the main body of the table, after the alkaline earth metals. At that time chemists believed these metals were divalent. Once the rare earths proved mostly trivalent and thorium tetravalent, Mendeleev moved cerium and thorium into group IV in 1871. Cerium was soon placed in a separate lanthanide series. Thorium stayed with group 4, because it resembled titanium and zirconium.

  Up to the late 19th century, chemists unanimously agreed that thorium and uranium were the heaviest members of group 4 and group 6. They considered the existence of the lanthanides in the sixth row a one-off fluke. In 1892, the British chemist Henry Bassett suggested a second extra-long row to hold known and undiscovered elements, treating thorium and uranium as analogues of the lanthanides. In 1913 the Danish physicist Niels Bohr published his model of the atom. It implied that the seventh row should fill f-shells before d-shells, just as the sixth row did with the lanthanides.

  Acceptance came slowly. The first transuranic elements showed that, from plutonium onward, elements have dominant +3 and +4 oxidation states like the lanthanides. So the actinides really were filling f-orbitals, and the transition-metal-like chemistry of the early actinides was the exception. In 1945, the American physicist Glenn T. Seaborg and his team discovered americium and curium. Seaborg then proposed the actinide concept. It made thorium the second member of an f-block series, not the heavier relative of hafnium.

  The chemistry still carries traces of the confusion. Thorium sits in the f-block, yet its outer electrons mostly occupy 6d orbitals, not 5f. Relativistic effects, which grow stronger near the bottom of the periodic table, cause this oddity. Thorium almost always loses all four valence electrons and appears with a +4 charge. In its ionization energies and redox potentials it resembles zirconium and hafnium more than cerium. Its +4 compounds are usually colourless or yellow.

  The metal's crystal structure gives away its true family. If thorium kept a pure 6d arrangement, it would be hexagonal close packed like titanium, zirconium and hafnium. Instead it is face-centred cubic, which can only be explained by involving the 5f states. That proves it is metallurgically a true actinide. In January 2021, researchers observed aromaticity in a cluster anion of 12 bismuth atoms stabilised by a single thorium cation at its center. The compound proved surprisingly stable.

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• 7. The Breeding Cycle

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  Thorium-232 cannot sustain a chain reaction on its own. It is fertile rather than fissile, because it can be converted into fissile uranium-233. In the thorium fuel cycle, slow neutrons strike thorium-232, which captures one to become thorium-233. That isotope decays in 21.8 minutes to protactinium-233, which then takes 27 days to become uranium-233. A rarer isotope, thorium-229, is fissionable. Its bare critical mass is estimated at 2839 kilograms, dropping to 994 kilograms with steel reflectors.

  The arithmetic of neutrons is tight. The fission of uranium-233 yields 2.48 neutrons on average. One neutron keeps the reaction going and one more breeds a new atom from the fertile material. That leaves a margin of 0.45 neutrons, about 18% of the neutron flux, for losses. The cycle parallels the uranium cycle in fast breeder reactors, where uranium-238 becomes plutonium-239.

  Supporters point to several advantages. Uranium-233 has a 92.0% chance of fission upon neutron capture, against 85.5% for uranium-235 and 73.5% for plutonium-239. A single neutron capture by uranium-238 produces transuranic waste alongside plutonium-239. Thorium-232 produces that waste only after five captures, which never happens to 98-99% of its nuclei. Thorium dioxide also has a higher melting point and thermal conductivity than uranium dioxide, plus a lower thermal expansion. It does not oxidise into a less dense compound as uranium dioxide does. Thorium is particularly suitable as fertile material in molten salt reactors.

  The disadvantages are just as specific. Used fuel is difficult and dangerous to reprocess, because many daughters of thorium-232 and uranium-233 are strong gamma emitters. Every route to uranium-233 leaves some uranium-232, which decays into the strong gamma emitter thallium-208. That makes uranium-233 easy to detect and dangerous to handle. The impracticality of separating it also limits its use for proliferation. Protactinium-233, with its 27-day half-life, soaks up neutrons as a poison. Molten salt reactors therefore extract it from the active zone during operation. Protactinium-231, with a half-life of 32,760 years, contributes to the long-term radiotoxicity of spent fuel.

  The International Nuclear Energy Agency's critics are not the only skeptics. The Nuclear Energy Agency predicts that the thorium cycle will never be commercially viable while uranium remains abundant, a situation it says may persist "in the coming decades". Mastering the irradiation and processing of thorium-232 also demands more advanced technology than the uranium and plutonium cycle. One country has stronger reasons than most to look past uranium.

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• 8. Reactors and Bombs

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  In 1943, the Manhattan Project contracted two private companies, Union Carbide and Chevron, to quietly survey uranium and thorium deposits around the world. The focus was uranium, but thorium joined the survey early. Monazite sands turned up in Brazil, the Netherlands East Indies and Travancore in India. The project pursued none of them.

  Thorium also appeared in a wartime design problem. When gram quantities of plutonium were first made, the minor isotope plutonium-240 turned out to undergo significant spontaneous fission. That threatened the viability of a gun-type plutonium weapon. While the Los Alamos team worked on an implosion design, the Chicago team looked for reactor solutions. Eugene Wigner proposed using the contaminated plutonium to drive the conversion of thorium into uranium-233 in a special converter reactor. Concerns about contamination from uranium-232 were voiced. Implosion progressed well enough that the converter was never developed. Its design still had enormous influence. It was the first detailed description of a highly enriched, water-cooled, water-moderated reactor, similar to later naval and commercial power reactors.

  During the Cold War, the United States explored thorium-232 as a source of uranium-233 for a nuclear bomb and fired a test bomb in 1955. It concluded that a uranium-233 bomb would be very potent. But it offered few sustainable technical advantages over the uranium-plutonium bombs of the day, partly because pure uranium-233 is difficult to produce. Thorium metal also went into the radiation case of at least one deployed American design, the W71.

  The earliest thorium-based reactor was built in 1962 at the Indian Point Energy Center in Buchanan, New York. Progress stayed slow in most countries because uranium was relatively abundant. In the 20th century, three reactors were built in India and twelve elsewhere. India's case is different. It has the largest estimated thorium reserves in the world and only sparse uranium. In the 1950s it targeted energy independence through a three-stage nuclear power programme.

  Alvin Radkowsky of Tel Aviv University in Israel was head designer of the Shippingport Atomic Power Station in Pennsylvania, the first American civilian reactor to breed thorium. He founded a consortium to develop thorium reactors. It drew in Raytheon Nuclear Inc. and Brookhaven National Laboratory in the United States, and the Kurchatov Institute in Russia. The International Atomic Energy Agency began large-scale research on thorium reactors in 1996. A year later the United States Department of Energy started its own.

  In the 21st century, thorium's potential for reducing nuclear proliferation and its waste characteristics revived interest in the fuel cycle. In 2009, the chairman of the Indian Atomic Energy Commission said that India has a "long-term objective goal of becoming energy-independent based on its vast thorium resources." India has projected meeting as much as 30% of its electrical demand with thorium-based nuclear power by 2050. In February 2014, the Bhabha Atomic Research Centre in Mumbai presented its design for the Advanced Heavy Water Reactor, which burns thorium in its fuel core.

  China may be the first to have tried to commercialise the technology. On the 16th of June 2023, its National Nuclear Safety Administration licensed the Shanghai Institute of Applied Physics, part of the Chinese Academy of Sciences, to operate TMSR-LF1. This is a 2 MWt liquid-fuel thorium molten salt experimental reactor, completed in August 2021. China is believed to hold one of the largest thorium reserves in the world. The exact size has not been disclosed, but it is estimated to be enough to meet the country's total energy needs for more than 20,000 years.

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